A spark plug used in an internal combustion engine or the like, for example, includes a center electrode extending in a direction of an axis, a tubular insulator disposed at the outer circumference of the center electrode, a tubular metallic shell disposed at the outer circumference of the insulator, and a ground electrode with a base end joined to the front end portion of the metallic shell. Further, the ground electrode is bent at an approximately center thereof such that the front end portion of the ground electrode faces the front end portion of the center electrode. A spark discharge gap is formed between the front end portion of the center electrode and the front end portion of the ground electrode.
In recent years, from the aspect of environmental protection, to obtain sufficient output while achieving low displacement, a high-compression and high supercharging engine may be employed. With such engine, a vibration applied to the ground electrode during operation of the engine tends to be large. Accordingly, breakage may occur at a flexed portion of the ground electrode where stress due to vibration is especially concentrated.
Therefore, to prevent breakage of the ground electrode, a technique that eliminates the flexed portion and makes the ground electrode a straight bar (straight) is proposed (for example, see JP 2003-59618 A or the like). A technique that increases the diameter of crystal grains at the flexed portion of the ground electrode to prevent the breakage of the ground electrode is known (for example, see JP 2005-339864 A or the like).
However, with the technique described in the above-described JP 2003-59618 A, the ground electrode comes closer to the center electrode not only at the front end portion but also at the middle portion. Hence, the presence of the ground electrode inhibits growth of a spark generated at a spark discharge gap, resulting in reduced ignitability.
With the technique described in the above-described JP 2005-339864 A, the stress applied to the flexed portion of the ground electrode due to vibration is still large. The breakage of the ground electrode may not be sufficiently prevented.
The present invention has been conceived to solve the above-mentioned problems. An advantage of the invention is a spark plug in which the breakage of the ground electrode or the like can further reliably be prevented while achieving superior ignitability.